Material removal may be accomplished in many ways. One common method involves bringing a rotating tool bit into contact with the material that is to be removed. For example, in a dental procedure, a dentist may operate a drill holder or handpiece with a tool bit mounted therein. In this application, the bit is referred to as a dental bur or simply a bur and has a surface designed to remove tooth material in a controllable and ideally painless manner. The handpiece is essentially a specially designed drill or rotary machine. The handpiece may be powered or driven in a variety of ways, for example, by compressed air that passes through an air turbine or by electricity via an electric motor such that a bur that is chucked or otherwise held in the handpiece may rotate at rates that may exceed 400,000 rpm.
While a bur may be of a single piece construction, the bur is often an assembly of multiple parts in order to address manufacturing costs and performance criteria. For example, the bur may be a two-part construction that generally includes a head joined to a shank or shaft. The head is designed to remove the targeted material and has an abrasive surface or cutting edges, like teeth or blades, for this purpose. In operation, when the bur is rotated and the cutting edges of the head are brought into contact with a softer material, the edges cut or remove the material. Again, referring to a dental bur as an example, the dentist may use the bur to remove portions of a tooth, such as tooth enamel, during endodontic therapy by bringing the rotating head in contact with the tooth.
To efficiently remove material at a high rate, the blades must remain sharp and penetrate the surface of the material. In this regard, the head of the bur is often made of a material that is harder than the material being removed. The material of choice is metal carbide, like tungsten carbide, though in some applications, diamond impregnated or electrodeposited diamond cutting edges may be used.
The shank functions to transfer the rotation from the handpiece, to the head as well as space the head a distance from the handpiece. Where the bur is constructed of two separate parts, the shank may be made of a low-cost, machinable, high-strength material, for example, steel.
The carbide piece, prior to joining and machining the cutting edges therein, is often referred to as a pill. With reference to FIGS. 1A and 1B, one technique that is used to join a pill 10 to a shank 12 to make a bur 14 includes forming a butt joint 16 between the two parts. As shown in FIGS. 1A and 1B, the butt joint 16 involves bringing two flat surfaces together, one flat surface on the pill 10 and one on the shank 12, and either welding or brazing the interface between the two flat surfaces to form the butt joint 16. Another joint configuration (not shown) that has been implemented to join the pill and the shank includes forming a recess in the pill and inserting a portion of the shank in the recess and brazing or welding the two together.
However, each of these constructions has its limitations. For one, while the butt joint is a simple, easily formed joint, a bur having a butt joint often breaks under loads encountered during material removal. That is, the joint is often the weak point in this construction, and sometimes limits the useful life and effectiveness of the bur. In particular and with reference to FIG. 1B, a lateral load L (arrow 18) or moment that is placed on the head or pill 10 of the bur 14 during use places surfaces of the bur 14 in tension (represented by σ1) and, consequently, the joint 16 is also at least partly in tension. Thus, the stresses caused by the applied load L tend to pull the joint 16 apart. These crack opening stresses (represented by σ2) are of the exact same magnitude as the tensile stress σ1.
Another joint configuration includes forming a recess in the pill in to which the shank is tilted and joined. However, this joint configuration is costly to machine and often requires specially adapted joining techniques that are generally not practical for manufacturing environments. For example, where the pill is made of tungsten carbide, machining of the recess in the pill usually requires diamond impregnated or other costly drill bits because metal carbides are well known for their extreme hardness. Consequently, machining the recess in a pill made of an extremely hard material adds significant costs to the manufacturing process. Furthermore, this technique may not provide a joint capable of resisting normal stresses encountered during use of the bur.
Consequently, there is a need for a bur that addresses the aforementioned problems. In particular, what is needed is a bur that is both strong and manufacturable in a cost effective manner.